1. Field of the Invention
The present invention relates to an automatic retrieval method of a wavelength-division multiplexed (WDM) ring network to the normal state after recovery of a failure. More specifically, the present invention relates to an automatic retrieval method of a WDM ring network to the normal state after recovery of a failure by inducing a lasing by making the gain of the closed loop larger than 1 using the fact that the recovered section in the ring network forms a closed loop immediately after the recovery of the failure.
2. Description of the Related Art
The development of wavelength-division multiplexing (WDM) technology and high speed electronic devices are enabling the era of ultra-high speed optical communication whose transmission capacity exceeds several Tb/s, and due to this extraordinary increase in the speed of communication networks, the importance of reliability in an optical communication network has newly been surfaced.
For example, a 400 Gb/s optical transmission system allows 5 million simultaneous calls through a pair of optical fibers; a communication failure of such system can cause chaos for a city or a nation as a whole.
Accordingly, it is very important to improve the reliability of a communication network along with the increase in the network capacity. In order to improve the reliability, a communication network should be equipped with a self-healing function.
Since a WDM communication network can transmit data or information without O/E or E/O conversion, it improves the transparency and reliability of the network as well as it can increase the transmission capacity.
There are several types of WDM networks that have self-healing capability. They are two-fiber unidirectional ring network, four-fiber bi-directional ring network, all-optical mesh network, and so forth.
A ring network is the simplest topology in which any pair of nodes has two separate paths between them. This makes a ring network flexible to failures. The WDM self-healing ring networks can provide consistent communication service through extra optical links by automatic restoration even if a failure occurs in the network.
FIG. 1 shows an example of four-fiber bi-directional WDM self-haeling ring network according to a prior art.
The network consists of four nodes 14a, 14b, 14c, 14d in total. Each of the nodes 14a, 14b, 14c, 14d is connected to working fiber links 10 and protection fiber links 12. Also, each of the nodes 14a, 14b, 14c, 14d is composed of an Add/Drop Multiplexer (ADM) for transmission (Add) and reception (Drop), space switches (SW), optical tap couplers (C) for monitoring of the state of the working fiber link 10, photodetectors (PD) for failure detection and control circuits (CC) for passing communication signals from the working fiber link 10 to the protection fiber link 12.
The working fiber link 10 is used when the network is in the normal state and the protection fiber link 12 is used when a failure occurs in the network.
If a failure such as a fiber cut occurs in the working fiber link 10, then a node switch (SW) adjacent to the failed fiber link 10 changes its state and detours the signal from the working fiber link 10 to the protection fiber link 12.
Previously, an automatic retrieval method using xe2x80x9cbridge and switch methodxe2x80x9d after recovery of the failure is proposed (Tesuya Miyazaki and Shu Yamamoto, A proposal of the optical switch circuit architecture with bridge and switch function for WDM four-fiber ring network, Third OptoElectronics and Communication Conference (OECC 98) Technical Digest Series, paper 16A4-4, July, 1998). However, it has a complex structure and requires additional components such as switch, optical power splitter, photodetectors, and electrical control circuits. They bring additional signal loss. Also, it can be used only for the four-fiber bi-directional WDM self-healing ring network.
The present invention is designed to overcome the above problems of the prior art. The object of the invention is to provide an automatic retrieval method of a WDM ring network to the normal operation condition after recovery of a failure by using a lasing in a closed loop, which is made of a working fiber and a protection fiber between two adjacent nodes where the loop gain of the closed loop should be larger than unity.
In order to accomplish the above object, the present invention provides an automatic retrieval method of a WDM ring network to the normal operation condition after recovery of a failure that comprises; a detection step where the photodetectors in the nodes detect a failure that occurs during the normal operation state, a control step where the control circuit controls the space switches in the node adjacent to the failed fiber section, a recovery step where the signals which have been transmitted along the working path is bypassed along the protection fiber, a closed-loop formation step in which a closed loop is formed with the working fiber and the protection fiber, a lasing step in which a lasing arises by making the loop gain of the newly formed closed loop exceeds 1, a retrieval step where the transmission signal returns to the working fiber link by monitoring the optical power in the failed section.